1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to wireless communication, and more particularly, to transmitting and receiving a digital signal using an orthogonal frequency division multiplexing (OFDM) communication system.
2. Description of the Related Art
Recently, active research has been carried out on cognitive radio technology and the use of frequency resources for efficient wireless communication. FIGS. 1A and 1B are diagrams for describing cognitive radio technology. Referring to FIG. 1A, generally, frequency resources available for wireless communication are allocated in a manner so as not to overlap with a variety of wireless communication standards. Therefore, communication with a device, which communicates according to a certain standard, may not be possible if all the channels allocated to the corresponding standard are being used, although channels allocated to other communication standards are not being used.
Cognitive radio technology searches available wireless channels according to regions and time in order to use available channels. As illustrated in FIG. 1B, the available channels can be searched for and used regardless of time and frequency bands.
FIGS. 2A through 2C are diagrams for describing OFDM communication.
FIG. 2A is a block diagram of an OFDM transmitting apparatus. Referring to FIG. 2A, when a digital signal is input, a serial-to-parallel (S/P) converter 21 splits the digital signal into a plurality of signals so as to input the signals to an inverse fast Fourier transformation (IFFT) device 22. In FIG. 2A, it is assumed that three-point IFFT is used. The IFFT device 22 performs IFFT on the input signals. When the IFFT is completed, digital signals corresponding to a plurality of subcarriers are generated. Since the IFFT and a FFT are well known and are disclosed in a variety of documents, detailed descriptions thereof will be omitted.
A parallel-to-serial (P/S) converter 23 combines the digital signals output from the IFFT device 22 and then converts the signals into a serial signal. A digital-to-analog (D/A) converter 24 converts a digital signal output from the P/S converter 23 into an analog signal. A mixer 25 performs frequency up-conversion using carriers which have radio frequencies (RFs).
FIGS. 2B and 2C are frequency domain graphs illustrating signals output from {circle around (1)} and {circle around (2)} of the OFDM transmitting apparatus illustrated in FIG. 2A. As illustrated in FIG. 2B, the subcarriers at baseband generated by the IFFT are modulated into RF signals at a frequency band of the carriers by the mixer 25 of FIG. 2A and then the modulated signals are transmitted externally.
In the above described OFDM system, although available frequency resources are searched for using cognitive radio technology, the frequency resources cannot be enabled if an available frequency bandwidth is less than the bandwidth of subcarriers.